O’Hare Noise Compatibility Commission

In 1996, Chicago created the O’Hare Noise Compatibility Commission (ONCC) as a policy-making group to oversee noise management efforts around O’Hare International Airport. The ONCC participates in the planning of noise mitigation projects to be implemented in the O’Hare area; oversees an effective and impartial noise monit​oring system; and advises the City of Chicago on O’Hare-related noise issues.

The ONCC is comprised of representatives of communities located within the O’Hare area. This means that decisions about how noise reduction money is spent will reflect the concerns of the communities that are most affected by aircraft noise.

For more information about the ONCC, including its membership, committee work, meeting schedule and accomplishments, please visit the ONCC website at www.oharenoise.org.

Sound and Noise

Sound is created by a vibrating source that induces vibrations in the air. The vibration produces alternating bands of relatively dense and sparse particles of air, spreading outward from the source like ripples on a pond. Sound waves dissipate with increasing distance from the source. Sound waves can also be reflected, diffracted, refracted or scattered. When the source stops vibrating, the sound waves disappear almost instantly and the sound ceases.

Sound can be defined in terms of three components:

1.Level (amplitude)

The level of sound is measured by the difference between atmospheric pressure (without the sound) and the total pressure (with the sound). Amplitude of sound is like the relative height of the ripples caused by the stone thrown into the water. Although physicists typically measure pressure using the linear Pascal scale, sound is measured using the logarithmic decibel (dB) scale. By definition, a 10 dB increase in sound is equal to a tenfold (10X) increase in the mean square sound pressure of the reference sound.

2.Pitch (frequency)

The pitch (or frequency) of sound can vary greatly from a low-pitched rumble to a shrill whistle. Consider the analogy of ripples in a pond; high frequency sounds are vibrations with tightly spaced ripples, while low rumbles are vibrations with widely spaced ripples. The rate at which a source vibrates determines the frequency. The rate of vibration is measured in units called hertz (Hz) – the number of cycles, or waves, per second. The ability to hear a sound depends greatly on the frequency composition. Humans hear sounds best at frequencies between 1,000 and 6,000 Hz.

3.Duration (time pattern)

The duration of sounds – the patterns of loudness and pitch over time – can vary greatly. Sounds can be classified as continuous like a waterfall, impulsive like a firecracker, or intermittent like aircraft overflights. Intermittent sounds are produced for relatively short periods, with the instantaneous sound level during the event roughly appearing as a bell-shaped curve. An aircraft noise event is characterized by the period during which it rises above the background sound level, reaches its peak, and then recedes below the background level.

Click here to see a 'Comparison of Sound' exhibi​t for both common indoor and common outdoor sound levels. The smallest detectable change by a human ear is +/- 1 dB (laboratory setting). A change of +/- 3 dB is noticeable to most people. Adding two equally like sounds adds a 3 dB increase.

Day-Night Average Sound Level (DNL) and Federal Requirements

The DNL metric describes the total noise exposure during a given period. DNL can only be applied to a 24-hour period. In computing DNL, an extra weighting of 10 dB is assigned to any sound levels occurring between the hours of 10:00:00 p.m. and 6:59:59 a.m. This penalty is intended to account for the greater annoyance that nighttime noise is presumed to cause for most people. Recalling the logarithmic nature of the dB scale, this extra weight treats one nighttime noise event as the equivalent to ten daytime events of the same magnitude.

For this reason DNL values are strongly influenced by the loud events. For example, 30 seconds of sound of 100 dB, followed by 23 hours, 59 minutes, and 30 seconds of silence would compute to a DNL value of 65 dB. If the 30 seconds occurred at night, it would yield a DNL of 75 dB.

DNL is the standard metric used for environmental noise analysis in the U.S. This practice originated with the USEPA’s effort to comply with the Noise Control Act of 1972. The USEPA designated a task group to “consider the characterization of the impact of airport community noise and develop a community noise exposure measure.” The task group recommended using the DNL metric. The USEPA accepted the recommendation in 1974, based on the following considerations:

The measure is applicable to the evaluation of pervasive, long-term noise in various defined areas and under various conditions over long periods of time.

The measure correlates well with known effects of the noise environment on individuals and the public.

The measure is simple, practical, and accurate.

Measurement equipment is commercially available.

The metric at a given location is predictable, within an acceptable tolerance, from knowledge of the physical events producing the noise.

Soon thereafter, the Department of Housing and Urban Development, Department of Defense, and the Veterans Administration adopted the use of DNL. At about the same time, the Acoustical Society of America developed a standard (ANSI S3.23-1980) which established DNL as the preferred metric for outdoor environments. This standard was reevaluated in 1990, and the same conclusions were reached regarding the use of DNL (ANSI S12.40-1990).

In 1980, the Federal Interagency Committee on Urban Noise (FICUN) met to consolidate Federal guidance on incorporating noise considerations in local land use planning. The committee selected DNL as the best noise metric for this purpose, thus endorsing the earlier work of the USEPA and making it applicable to all Federal agencies.

In response to the requirements of the Aviation Safety and Noise Abatement Act of 1979 (ASNA) and the recommendations of FICUN and USEPA, the Federal Aviation Administration (FAA) established DNL in 1981 as the single metric for use in airport noise and land use compatibility planning. This decision was incorporated into the final rule implementing ASNA, specifically Part 150 of Title 14 of the Code of Federal Regulations (14 CFR Part 150), in 1985.

In the early 1990s, Congress authorized the creation of a new interagency committee to study airport noise issues. The Federal Interagency Committee on Noise (FICON) was formed with membership from the USEPA, the FAA, the U.S. Air Force, the U.S. Navy, the Department of Housing and Urban Development, the Department of Veterans Affairs, and others. FICON concluded in its 1992 report that Federal agencies should “continue the use of the DNL metric as the principal means for describing long term noise exposure of civil and military aircraft operations.” FICON further concluded that there were no new sound descriptors of sufficient scientific standing to substitute for the DNL cumulative noise exposure metric.

In 1993, the FAA issued its Report to Congress on Effects of Airport Noise. Regarding DNL, the FAA stated, “Overall, the best measure of the social, economic, and health effects of airport noise on communities is the Day-Night Average Sound Level (DNL).”

Introduction to Noise

Click here to review a background paper​ regarding the principles of noise, noise analysis and modeling, as well as the preparation of airport noise exposure maps and how the estimates of noise impacts inside a 65 DNL noise contour are determined.

Glossary of Terms and Acronyms

Noise Contours

Noise contours are computed using the Integrated Noise Model (INM). The INM was developed under the guidance of the FAA and is the only model generally approved by the FAA. INM has been FAA's standard tool since 1978 for determining the predicted noise impact in the vicinity of airports. The noise pattern calculated by the INM for an airport is a function of several factors, including: the number of aircraft operations during the period evaluated, the types of aircraft flown, the time of day when they are flown, the way they are flown, how frequently each runway is used for landing and takeoff, and the routes of flight used to and from the runways. Substantial variations in any one of these factors, when extended over a long period of time, may cause marked changes to the noise pattern.

The current noise contour is called the OMP Build-Out Noise Contour, and represents the future noise impacts estimated when the build-out of the OMP occurs, currently scheduled for the year 2020. The OMP Build-Out Noise Contour was approved by the FAA in 2005. The Chicago Department of Aviation (CDA) will use this noise contour for residential sound insulation until the time of OMP build-out.

Aircraft Noise Footprints

An aircraft noise footprint illustrates the noise levels produced by a specific aircraft type during landing and takeoff. The noise footprint represents the maximum sound level experienced on the ground as the aircraft flies over.

O’Hare Aircraft Operations

Community Outreach Vehicle

The CDA and the ONCC disseminate a variety of information related to aircraft noise issues through its community outreach programs. Some of that information is aboard the CDA Community Outreach Vehicle (COV).

The COV is available to travel to community events and schools throughout the year. There are video presentations and computer demonstrations aboard the COV that explain, among other things, the Airport Noise Management System and noise footprints of different types of aircraft. The COV is also used as a learning tool. It is available to travel to schools located within the City of Chicago and surrounding suburbs to discuss noise management, noise mitigation and aviation career possibilities.

The COV is powered by compressed natural gas (CNG) and is equipped with an ADA-compliant automatic hydraulic wheelchair lift.

Anyone interested in having the COV visit a village hall, community event, or school should contact Assistant Commissioner Aaron Frame at the Chicago Department of Aviation, 773-686-3563 or aaron.frame@cityofchicago.org​.